1. Field of the Invention
The present invention relates to a method of buffering video data in a video compression system, and more particularly to a buffering method for transmitting a video bit stream compressed on the basis of an international standard.
2. Description of the Prior Art
In the international standardization of mobile communication techniques centering around a wideband code division multiplexing access (W-CDMA) technique, a mobile video data communication standard has been proposed recently by a recommendation of the International Telecommunications Union Telecommunications Standard (hereinafter xe2x80x9cITU-Txe2x80x9d) on a television transmission line and audio broadcast transmission line.
In such international standards on the mobile video data communication, a video buffering technique is commonly used.
The video buffer temporarily stores the compression-coded variable-rate video data for a predetermined time period prior to outputting the data at a rate matching the communication channel. If a communication channel is a variable bit rate (VBR) type in which data is processed regardless of a variation in the rate of compressed video data, no buffering is required. However, in most wire/wireless digital communications, the communication channels have a constant bit rate (CBR). As a result, a video buffering technique capable of controlling the variable-rate video data adaptively to a constant-bit rate communication channel is required.
Also, even in the case where a communication channel is the variable bit rate type, a network is congested upon inputting video data of a rate higher than the maximum variable rate of the communication channel. In order to prevent such a congestion, a video buffering technique which limits the rate of video data or a rate control technique similar thereto is required.
FIG. 1 is a block diagram showing a conventional video data compression apparatus in the related art. This video data compression apparatus is a basic model defined in an international standard on video compression coding, or ITU-T H.263, MPEG-1, MPEG-2 and MPEG-4. As shown in FIG. 1, the video data compression apparatus comprises a discrete cosine transform (DCT) unit 1 inputting macro block-unit video data before compression, a quantizer 2, an inverse quantizer 3, an inverse discrete cosine transform (IDCT) 4, and a frame memory 5 for video storage. The video data compression apparatus further comprises a variable length coder (VLC) 6 converting video data quantized by the quantizer 2 into variable-length data, a video multiplexer 7 multiplexing the variable-length codes from the VLC 6, and a buffer 8 transmitting the compressed video data from the video multiplexer 7 externally via the channel.
The buffer 8 acts as a single port for input/output of video data. The buffer 8 has an occupancy which is periodically monitored on the basis of an address difference between a read pointer and a write pointer. Also, the buffer 8 notifies a video rate controller 9 coupled with the quantizer 2 of the monitored occupancy. The buffer 8 forms a feedback loop with the video rate controller 9. Also, the buffer 8 inputs the compressed variable-rate variable-length video data and outputs constant-rate variable-length video data.
The video rate controller 9 judges the occupancy O (k) of the buffer 8 and transfers a proper quantization coefficient Q (k) to the quantizer 2 according to the judged result.
The quantization coefficient Q (k) is a quantization step size of any one of 31 integers from 1 to 31. When the quantization coefficient Q (k) is large, the amount of output data from the quantizer 2 is reduced. If the quantization coefficient Q (k) is small, the amount of output data from the quantizer 2 is increased.
Variable-rate and variable-length video data, compression-coded by the quantizer 2, the VLC 6 and the video multiplexer 7, are serially input to the buffer 8. From the buffer 8, the data is serially output at a constant-rate variable-length video data in the input order. FIG. 2 shows an internal structure of a conventional buffer 8 in FIG. 1.
As shown in FIG. 2, the conventional buffer is configured in a first-in first-out (FIFO) manner where compressed video data are serially input and output in the input order. Such a FIFO memory is subjected to a serial input/output control operation which does not allow a random access operation. This FIFO memory usually has functional pins usable to indicate an occupancy full or occupancy empty state.
In the conventional buffer as discussed above, an overflow or underflow may occur due to a difference between the amounts of input/output data because it employs a FIFO memory with a serial input/output function. Also, because the step size of the quantizer is a main factor for controlling the data amounts, the buffer may perform a faulty operation even under a correct control. In particular, upon occurrence of an overflow, the buffer is unable to store new input video data, resulting in a disconnection with a communication channel. Recovering of the connection requires resynchronization and a considerable amount of processing time.
The above problem may be compensated by increasing the size of the buffer to reduce the faulty operation rate thereof. However, this causes a transfer delay, resulting in a degradation in service quality.
Another issue is to prevent a degradation in picture quality when the flow amount to the buffer is increased due to an abrupt increase in the video data amount. To minimize the degradation effect, the degradation in the picture quality should be distributed to the surroundings. However, because variation cannot be applied to video data previously stored in the buffer, the picture quality degradation appears mostly on new input video data. This causes an abrupt variation in picture quality and thus, a degradation in service quality.
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide a buffering of video data in a video compression system, in which the memory locations are divided according to variable-length elements to define the boundary between input variable-length codes in order to permit individual accesses.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, a video data buffering in a video compression system comprises the step of performing a discrete orthogonal transform operation with respect to the input video data in the block units and storing the resultant coefficients in the unit of discrete orthogonal transform components.